The successful operation of satellite communication networks, such as demand assignment (contention) systems, depends upon the ability of the receiver equipment at the respective stations of the network to be accurately tuned to the incoming signal from another site. In a demand assignment communications scheme, messages from respective contention participants of the network are transmitted in a burst format, in which a respective station's carrier is turned on for an abbreviated period, or time slot, during which a message (e.g. data packet) is transmitted and then turned off until that station has a new message to send.
Because of the intermittent nature of burst mode communications and carrier distortion introducing characteristics of the satellite channel, the ability to provide an accurate demodulation reference frequency for signal recovery becomes a significant problem. One way to solve the problem is to provide each station with a high precision oscillator which monitors an effectively perfectly stable pilot frequency, that has been transmitted from a master site, in order to determine frequency offset through the satellite link and to use this information to accurately control the characteristics of its burst carrier, so that the message, when received at the master site, will be effectively precorrected, permitting demodulation and data recovery. The problem with this approach is two-fold: on the one hand, the cost of the equipment at each burst-sourcing site is increased substantially by the provision of the high precision reference oscillator. In addition, because the network uses a pilot tone for the purpose of frequency correction, the capacity of the satellite link (an extremely precious resource) for data transmission is reduced.
One proposal to eliminate part of the problem, namely to reduce the expense of the equipment (high precision oscillator) at the burst-sourcing site is described in U.S. Pat. No. 4,509,200 to Luginbuhl et al, entitled "Satellite Telecommunications System". Pursuant to the patented scheme, a high precision pilot tone oscillator is installed at the master or central station, the only purpose of which is to measure frequency offset (drift) through the satellite. By monitoring the pilot tone over a loop back to itself, the master station is able to measure the offset through the satellite, which, as pointed out above, must be corrected. A signal representative of the value of this measured offset or error is then transmitted as an information signal for use at each remote site. The remote site, which does not have the benefit of the high precision oscillator, extracts the data to properly tune itself. This operation presupposes that the remote site is properly tuned to begin with, something that the coarse oscillator used at the remote site cannot guarantee. Consequently, the proposed procedure is questionable at best. Of course, due to the fact that the patented scheme dedicates part of the satellite link to the offset-controlling pilot tone, the resource occupancy problem still exists.